Usually, only measurements of steady
phase characteristics are primarily addressed. However, a new testing
method equally supplies additional information on the dynamic phase behavior
of an electric motor or an application. The additional information can
indicate defects such as vibrations and excessive noise levels, usually
not achieved on common production and laboratory quality control means.
Newton’s 2nd law for rotating devices describes steady and dynamic
phases of any given rotating device movement. Each of the phases can be
determined for a test measurement:
T = Tload + J dw/dt
T (load) represents a motor or application under a conventional dynamometer
test on a steady phase (stabilized on a constant-speed, no-acceleration
point and subjected to certain load torque value). In order to project
the Torque vs. Speed valuable performance curve, dynamometer
tests have to be repeated over several times to cool down heated motors
to test them again in order to ensure consistent motor performance and
T = J dw/dt
J dw/dt represents the motor under the new test on a dynamic phase
(during the acceleration duration, where it is not subjected to any
only using the motor’s internal inertia J). The Torque vs. Speed
valuable performance curve and other motor curves (amps, volts, pin, pout,
efficiency, and power factor, etc.) are accurately projected by the new
testers in seconds based on thousands of samples taken on the motor’s
acceleration process from zero to maximum “no load” speed.
The load on the motor is only its own momentum of inertia. From the recorded
values, software calculates the complete speed range, including power input,
power output, efficiency, and direction of rotation.
Additional unique tests to reveal noise, vibrations, assembly misalignment,
and bad bearings due to electrical and mechanical sources, include Torque
and Speed Spectrum, Torque Speed Oscillations, and Friction Torque. The
method is highly valuable for production verifying quality control of the
motor’s complete parameters. It is also valuable for R&D and
quality laboratory applications, easily testing dozens of motors per day
by a single operator, detecting defects on early stages, and projecting
various built-in reports.
The determination of the complete characteristics of an electric motor
over the whole speed range offers advantages. Checking motors only at the
maximum “no load” speed satisfies most motor manufacturers’ present
requirements. However, this has only minimal value. It shows only that
the motor is capable of rotating. In most cases, faults do not evidence
at maximum no-load speed, but in the lower speed range. The new testing
system conducts measurements on complete motor’s performance curves,
record data, and displays information within seconds.
In addition, the high resolution of the test results allows simultaneous
viewing and analysis of the motor or applications dynamic processes. Very
often, no irregularities are recognizable in the “steady” characteristics
of a motor, while they become visible by measuring transient behavior.
An example from real life illustrates this.
Detecting Defects in a Production Batch of Three Motors
A leading European manufacturer of household appliances intended to deliver
a new appliance model to the market. The prototype tested to specification
with its drive installed fully satisfied. The manufacturer was ready for
mass production. Production began, and the motors bought from an outsourced
supplier were installed.
Unexpectedly, some motors were not as quiet as the prototype tested. A
check in the acoustic laboratory revealed differences. The appliance producer
then tested the three motors using the new testing method. The motors included
a good motor (1), a tolerable motor (2), and a bad motor (3), originally
from the same production batch.
The test recorded voltage, auxiliary voltage, capacitor voltage, main current,
auxiliary current, speed, torque, power input, power output, efficiency,
friction torque, torque speed oscillations, and torque and speed spectrum
The resulting power output, torque, and efficiency of the three motors
differed only slightly. The peak values for output and torque of the bad
motor were approximately 7 percent below those of the good motor. Since
the current input had changed and, therefore, also dropped the power input,
the measurement of the steady motor characteristics showed no significant
differences, explaining the reasons for the different acoustic behavior
of the motors.
When the recorded data was evaluated in a different way, more useful information
was gathered. Figure 1 shows the increase in speed of the three motors
over time during acceleration. It is clearly visible that the good motor
runs up steadily to its maximum speed, while the bad motor periodically
decelerates during acceleration. The tolerable motor shows similar behavior
to the bad motor, although less pronounced.
This instability also resulted in torque oscillations during acceleration,
as shown in Figure 2. Even the smoothly running, “good” motor
executes oscillations around the mean value, yet the oscillation amplitudes
of the unsteadily running bad motor are approximately five times as large.
Figure 3 shows that a similar picture was obtained at the point of maximum
speed. Oscillations around the mean value of the point of maximum speed
increase from the good motor to the tolerable motor, and again to the bad
motor, although the amplitudes are not as serious as in the acceleration
phase. A frequency analysis (Figure 4) shows the frequencies having the
largest torque amplitudes. The disturbance has electromagnetic reasons
with double line frequency of 100 Hz, as can be seen in Figure 4. If the
motors with excessive torque spectrum were to be used inside the appliance
application, it would result in an unpleasant audible effect.
It is concluded that the new testing technology easily identifies outstanding
motors with excessive performances and indicates the reasons for the noise
development. Similar investigations conducted on gear motors or complete
drives with various components check stability in transient and steady
state operation. Gear defects and bearings and brush problems can also
be recognized immediately.
Delivering motors of unsatisfactory quality as described above incurs considerable
cost for the appliance producers as well as for their electric motor supplier
(this includes returns of appliance products, disassembling products, tests
on appliance laboratory plants, returns of motors to their manufacturers,
tests on motor manufacturer plants, and delays in delivery and production).
Implementations of the new test capabilities on appliance and electric
motor plants, quality assurance entrance control, and R&D and assembly
lines, can avoid significant costs.
This information is provided by Alfred
K. Wunsch, EC ENERGOCONSULT and Ronen Schmerler, MEA
Testing Systems LTD, Netanya, Israel.